1. Field of the Invention
This invention relates to liquid level sensing. More specifically, the invention is a liquid level sensing system using a form of electromagnetic energy such as optical energy.
2. Description of the Related Art
Typical liquid level sensing systems that measure the amount of cryogenic liquid in a tank use differential pressure measurement, capacitance measurements, or measurements made with a series of temperature or thermal resistance sensing devices in a rake or array configuration. Differential pressure measurements only work well in non-flowing conditions. Capacitance measurements do not work well when used in thermal gradients and dynamic flow conditions. Thermal measurement systems utilize the thermal change between a cryogenic liquid's gas-to-liquid fluid phases, and are limited by thermal latency or the time it takes for the thermal sensing elements to respond to a temperature change. In addition, the surface boundary in cryogenic liquid storage (i.e., the ullage layer) transitions from liquid to saturated vapor to gas. The temperature of the saturated vapor layer is very close to that of the liquid layer. This makes it difficult for a temperature-based system to resolve the true liquid boundary in a tank. Still further, the accuracy and resolution of conventional liquid level sensing systems are determined by the number of sensing elements. Since the number of sensing elements is typically tied to individual data channels, substantial data acquisition systems are usually required to achieve accurate results.